In the past, gas heating appliances used a gas valve that was opened to supply combustion fuel to the appliance and closed to stop combustion fuel to the heater. High efficiency gas heating appliances use a forced air induction system to supply combustion airflow to the heater. In the past, a simple switch has been used to determine whether the combustion airflow was sufficient for safe operation. The switch did not measure the value of the combustion airflow, but rather merely determined whether airflow over a prescribed value was present. Typically, the combustion air blower motor did not have multiple speeds, but rather ran at a single speed. Therefore, if the switch detected insufficient airflow, the gas valve would be turned off to prevent unsafe operation rather and the heating appliance would cease operation. Past systems did not reduce gas flow or increase the inducer motor speed to attempt to supply more airflow.
Modern heaters increasingly incorporate modulating gas valves that provide two or more defined or infinitely controllable gas input rates for more efficient operation. The appropriate combustion airflow rate for safe operation is different for the different gas input rates and therefore, when an appliance has multiple defined or infinitely controllable gas input rates the control system will typically have the ability to select different speed taps of a combustion air blower motor (or to provide a variable current input to a variable speed motor) to supply one of a number of possible airflows. Typically, to assure safe operation, multiple pressure switches have been implemented to ensure that the appliance operates at a safe level of combustion airflow for the selected gas input rate.
However, the use of multiple pressure switches is expensive and does not allow a heater control system to determine the precise airflow that is being accomplished in the particular heater installation. By failing to determine the actual airflow, the most efficient and safe airflow for a given gas input level cannot be obtained. This is also important because different heater installations will have different lengths and configurations of inducer air supply and exhaust pipe, which will present different amounts of backpressure which, in turn, will cause different airflows at a given inducer fan speed.
Therefore, there is a need for a control system that can provide for efficient operation of a heater in a plurality of installation environments by determining the actual inducer airflow present.